Desalting plants which use membranes in the desalting process must monitor the membrane performance to ensure stable operation. The quality of prefiltered water delivered to the membranes of such plants largely determines the membrane performance and longevity. Feed water quality can be monitored using a "plugging factor" (PF) method which indicates the quantity of particulate matter present in the feed water to the desalting membranes. A manual test was developed in the early 1970s and has been generally accepted through the industry as a criterion for determining the fouling tendency of membrane feed water. In this regard, membrane manufacturers use this PF as an indication of desalting unit feed water quality.
The plugging factor test itself is a measurement of the percent decrease of flow rate through a 0.45 mm filter membrane with a constant pressure differential of 207 kPa (gauge) after 15 minutes. This can be represented by the formula ##EQU1## where the quantity t.sub.1 is the time period of the initial measurement and t.sub.2 is the time period of the measurement performed 15 minutes later. It is theorized that the decreased flow is due to particles plugging some of the pores of the filter membrane and accordingly, the resultant percentage of flow reduction is related to the percent of plugged pores. If the total number of pores in a new filter membrane and the total volume of fluid flow during the test are known, the number of plugging particles per unit volume can be calculated.
A manual system such as referred to above has obvious shortcomings particularly with respect to the considerable amount of operator time required, and attempts have been made to develop an automatic continuous plugging factor monitor. One such continuous plug factor monitor which has been evaluated employed a filter holder assembly comprising a pressure actuated upper filter head movable to an operative position wherein a membrane filter, supplied from a membrane tape feed roll by action of a puller motor, is captured between the upper filter head and a lower filter head or base. The sample water under test flowed through the membrane held by the filter holder assembly to a filtrate tank including level probes which extended into the tank and which were used in determining the time required for a standard predetermined volume (131 ml) to be measured out. Two such time period determinations were made, corresponding to the periods t.sub.1 and t.sub.2 of the PF formula discussed above, and a programmer was used to control sequencing of the operations of the off-on valves in the system, the puller motor for the membrane tape and the upper filter head actuator. Cycle and interval timers provided inputs to the programmer and an event recorder used in displaying the values t.sub. 1 and t.sub.2 was connected to the output thereof.
Continuous plugging factor monitors that have been developed to date possess several disadvantages which have limited their usefulness. These disadvantages include poor precision, particularly for plugging factors lower than 50%; poor system reliability (caused principally by breakage of filter tape); and limitations on calculation and recording of PF results.